Can siblings detect pregnacy?

Timing and truth. When a woman suspects or confirms she is pregnant, she might begin to act differently. Often she chooses to not tell anyone during the early weeks. However, based on her behavior change and then the development of the baby bump in her belly, family members become suspicious and curious. Determining the right moment to tell them is sometimes difficult. An experienced OB provider can help with that.
Maybe. Women who are nursing can sometimes become pregnant, and sometimes report that their nursing infant acts differently suggesting that the breast milk has changed. For older children, they may pick up on a different "vibe" you're unconsciously giving off.